Voltage references are available with fixed and adjustable reference voltage outputs. Adjustable output is set by a resistor divider connected to a reference pin. These references are either shunt (two-terminal) or series (three-terminal) types. These ICs usually come in families of parts that provide specific accurate voltages. Some families can have up to six different values with output voltages from 1.225V to 5.000V and tolerances ranging from ±0.25% to 2%. Initial output voltage accuracy and temperature coefficient are two of the more important characteristics. Among the characteristics that an ideal voltage reference should have are:
- Output voltage independent of temperature changes
- Output voltage independent of load current
- Output voltage independent of time.
The ideal voltage reference should also have:
- Perfect initial accuracy
- Current source and sink capability
- Low quiescent current (or power dissipation)
- Low noise
- Reasonable cost
However, these ideal characteristics are unattainable, so the designer must consider the following:
Reference Sources include zener diode usually used in two-terminal shunt devices. A major advantage of zeners is the wide range of voltages that their wide range, from 2V to to 200V. They also have a wide range of power handling capability, from several milliwatts to several watts. Disadvantages of zener diodes are that they may not be precise enough for high-precision applications. Also, their power consumption makes them a tough fit for low-power applications. Another concern with a zener reference source is the relatively high output impedance of some types.
Another reference source is the bandgap usually used in three-terminal series ICs. A bandgap voltage reference is a temperature independent voltage reference circuit widely used in integrated circuits, usually with an output voltage about 1.25 V, which is close to the theoretical 1.22 eV bandgap of silicon at 0 K.
Shunt references are similar to zener diodes in operation because both require an external resistor that determines the maximum current that can be supplied to the load. The external resistor also sets the minimum biasing current to maintain regulation. You should consider shunt references when the load is nearly constant with minimal power supply variations.
Series references do not require any external components and they should be considered when the load is variable and lower voltage overhead is important. They are also more immune to the power supply changes than shunt references.
Series references have some advantages over shunt types. Shunt references require a resistor that must be chosen to supply the maximum current demanded by the load. When the load is not operating at this maximum current, the shunt reference must always sink this current, resulting in high dissipation and shortened battery life. Series references do not require a current setting resistor and are specified to operate over a relatively large supply voltage, which depends on the output voltage option.
Temperature Drift is the change in output voltage due to the temperature change, expressed in ppm/°C. Buried zener type references typically have a lower temperature drift than bandgap voltage references. Temperature drift can be specified in several ways (slope, butterfly and box) but the most common way is the box method.
Output Voltage Temperature Hysteresis is the change in the output voltage at the reference temperature, usually 25°C, due to sequential but opposite temperature excursions, i.e., cold-to-hot and hot-to-cold. Negative effects can occur due to this effect because its amplitude is directly proportional the temperature excursions of the associated system. In some systems this parameter is not repeatable. This parameter is a function of the circuit and the semiconductor package. Hysteresis is given in ppm (parts per million).
Initial Accuracy is important in systems where calibration is impossible or inconvenient. Usually, it is accomplished by the calibration of the overall system. Initial accuracy is specified with fixed input voltage and no load current (for series type) or fixed bias current (for shunt type).
Long-Term Drift (LTD) affects the output of a voltage reference, which changes gradually with time. The largest change occurs in the first 200 to 500 hours. This parameter is important in high-performance applications or in applications where periodic calibration is not acceptable. Long-term stability data may be based on the observation over 1000 hours at room temperature. Therefore, if LTD is important it might require frequent calibration and also a circuit burn-in.
Power dissipation depends on the voltage and current required to maintain proper operating characteristics.
Output Noise is usually specified over two frequency ranges: 0.1 Hz to 10 Hz (peak-to-peak noise) and 10 Hz to 1 kHz (RMS noise). Noise can be important because it can reduce dynamic range of an acquisition system. High-resolution data acquisition systems may experience “dither” in the LSBs solely due to reference noise. Noise can be reduced by external capacitor filtering of a noise reduction pin.
Voltage Reference ICs
Analog Devices ADR3412/ADR3420/ADR3425/ADR3430/ADR3433/ ADR3440/ADR3450 are low cost, low power, high precision CMOS series voltage references, featuring ±0.1% initial accuracy, low operating current, and low output noise in an SOT-23 package. For high accuracy, output voltage and temperature coefficient are trimmed digitally during final assembly using patented DigiTrim® technology from Analog Devices Inc.
Low TC, low long term drift and low output voltage hysteresis help maintain system accuracy over time and temperature variations, potentially reducing the need for equipment recalibration. Also, the low operating current of the device (100 μA maximum) facilitates use in low power, battery operated devices.
These voltage references use a patented architecture to achieve high accuracy, low temperature coefficient (TC), and low noise in a CMOS process. Like all bandgap references, the references combine two voltages of opposite TCs to create an output voltage that is nearly independent of ambient temperature. However, unlike traditional band gap voltage references, the temperature-independent voltage of the references are arranged to be the base-emitter voltage, VBE, of a bipolar transistor at room temperature rather than the VBE extrapolated to 0 K (the VBE of bipolar transistor at 0 K is approximately VG0, the band gap voltage of silicon). A corresponding positive-TC voltage is then added to the VBE voltage to compensate for its negative TC.
The key benefit of this technique is that the trimming of the initial accuracy and TC can be performed without interfering with one another, thereby increasing overall accuracy across temperature. Curvature correction techniques further reduce the temperature variation. The bandgap voltage (VBG) is then buffered and amplified to produce a stable output voltage. The output buffer can source up to 10 mA and sink up to −3 mA of load current.
These CMOS ICs are available in a wide range of output voltages, all specified over the industrial temperature range of −40°C to +125°C.
Texas Instruments’ REF50xx is a family of low-noise, low-drift, high precision series voltage references. Excellent temperature drift (3 ppm/°C) and high accuracy (0.05%) are achieved using proprietary design techniques.
Fig. 1 shows a simplified block diagram of the REF50xx family that provides a very accurate voltage output. However, VOUT can be adjusted to reduce noise and shift the output voltage from the nominal value by configuring the trim and noise reduction pin (TRIM/NR, pin 5). The TRIM/NR pin provides a ±15-mV adjustment of the device bandgap, which produces a ±15-mV change on the VOUT pin.
A supply bypass capacitor ranging between 1 µF to 10 µF is recommended. A 1 µF to 50 µF, low-ESR output capacitor (CL) must be connected from VOUT to GND. It must be less than or equal to 1.5 Ω. The ESR minimizes gain peaking of the internal 1.2-V reference and thus the REF50xx allows access to the bandgap that reduces noise at the VOUT pin. Placing a capacitor from the TRIM/NR pin to GND in combination with the internal 1-kΩ resistor creates a low-pass filter that lowers the overall noise measured on the VOUT pin. A capacitance of 1 µF is suggested for a low-pass filter with a corner frequency of 14.5 Hz. Higher capacitance results in a lower cutoff frequency.
There are five different output voltage versions providing 2.048, 2.5, 4.096, and 4.5V. Each reference voltage is available in both standard and high-grade versions. They are offered in SO-8 packages and are specified from -40°C to +125°C.
Linear Technology’s LT6656 (Fig. 2 ) is a precision series voltage reference that draws less than 1μA of supply current and can operate with a supply voltage within 10mV of the output voltage. The LT6656 offers an initial accuracy of 0.05% and temperature drift of 10ppm/°C. The LT6656 can supply up to 5mA of output drive with 65ppm/mA of load regulation, allowing it to be used as the supply voltage and the reference input to a low power ADC. The LT6656 can accept a supply voltage up to 18V and withstand the reversal of the input connections. The LT6656’s output is stable with 1μF or larger output capacitance and operates with a wide range of output capacitor ESR.
This reference is fully specified for operation from -40°C to 85°C, and is functional over the extreme temperature range of -55°C to +125°C. The LT6656 is offered in the 6-lead SOT-23 and (2mm ×2mm) DFN Packages.
Available in three accuracy grades, the LM4030 is a precision shunt voltage reference with an initial accuracy at 0.05%. The LM4030 exhibits 75ppm thermal hysteresis and 40ppm long-term stability as well as immunity to board stress effects.
The LM4030’s design eliminates the need for an external stabilizing capacitor while ensuring stability with any capacitive load, thus making the LM4030 easy to use. It is available with fixed voltage options of 2.5V and 4.096V.
The LM4030 utilizes fuse and zener-zap reverse breakdown voltage trim during wafer sort to ensure an accuracy of better than ±0.05% (A grade) at 25°C and guaranteed temperature coefficient of better than 10 ppm/°C. Table 1 lists the LM4030’s specifications.
The MAX6330/MAX6331 combine a precision shunt regulator with a power-on reset function in a single SOT23-3 package. The shunt regulator consists of a pass device and a controlling circuit, as illustrated in Fig. 4. The pass device allows the regulator to sink current while regulating the desired output voltage within a ±1.5% tolerance. The shunt current range (ISHUNT) is 100μA to 50mA.
Both active-low (MAX6330) and active-high (MAX6331) push/pull output versions are available. The output voltage has ±1.5% tolerance. The MAX6330/MAX6331 operate over a wide shunt current range from 100μA to 50mA, and offer very good transient immunity.
A 3-pin SOT23 package reduces board space and improves reliability compared to multiple-IC/discrete solutions.
The pass transistor in the MAX6330/MAX6331 maintains a constant output voltage (VSHUNT) by sinking the necessary amount of shunt current. When ILOAD is at a maximum, the shunt current is at a minimum, and vice versa:
IIN = ISHUNT + ILOAD = (VIN - VSHUNT) / RS (1)
When choosing choosing the external resistor, consider:
- The input voltage range, (VIN)
- The regulated voltage, (VSHUNT)
- The output current range, (ILOAD)
The TS3431 (Fig. 5) is a programmable shunt voltage reference with guaranteed temperature stability over the entire operating temperature range (-40°C to +125°C). The output voltage can be set to any value between 1.24V and 24V with an external resistor bridge.
Available in SOT23-3 surface mount package, it can be used in application designs where space saving is critical. Table 2 lists the TS431 specifications.